The requirements set for data transmission systems continue to increase. This concerns especially wireless data transmission systems, such as cellular radio systems, which are required to provide more and more varied services, including different data and video services.
Traditionally, wireless data transmission systems have been used to transmit only speech. The increasing number of different services to be transmitted means especially in wireless systems that a system must be able to transmit signals with different capacities over the radio path, for example to transmit speech at the data rate of 8 kbps and data at the rate of 64 kbps. It is also necessary to transmit signals with different quality levels, which typically require different data rates simultaneously. A typical example is a video connection where the image requires a high data rate and a high quality level, but the voice can be transmitted at a lower data rate and with a lower quality level. Therefore, a data transmission system should be able to operate effectively in an environment where transmissions of several different data rates, quality levels and service types are forwarded.
It is typical of digital data transmission systems that signals are transmitted in frames and that the size of a frame is predetermined. When different data rates are used, it is not always possible to select for all data rates a frame which is exactly of the correct size and which has space for the symbols to be transmitted, but it is necessary to change the data rate so that the information to be transmitted can be forwarded in the available frames.
A data rate can be either increased or decreased, as required. The best known and the simplest method of increasing the data rate is repetition coding, wherein user data symbols are repeated as many times as there are free symbol positions in the frame. Correspondingly, the data rate is decreased in the simplest manner by removing user data symbols until the remaining symbols fit in the available frame. The problem in the prior art arrangement is how to select the symbols to be repeated or removed so that they are distributed evenly among the symbols to be transmitted.